One have to bear in mind that the JV is for the entire EL4322...

One have to bear in mind that the JV is for the entire EL4322 and not just the Vulcan target. It's a large area of 1800km2 and probably the last chance for Rio to get a decent footprint in the region. Almost all surrounding ground is locked up by majors. That fact alone could be enough for Rio to proceed with the JV. (It might be a bit expensive in this JV setup, i.e. obligatory drilling cost, but don't rule it out) Besides Tas already pulled the next target out of their hat. Which I'm sure we'll hear more about in the near term.

From the quarterly:

Further Work
'Tasman has identified targets for further drilling in the northern part of Vulcan and associated with both the north eastern “limb” of the Vulcan structure and the main southern part of Vulcan. Attention also needs to be given to evaluation of the very large more grassroots prospect between Vulcan and Titan.'